Conventional start-stop functions for motor vehicles are used with internal combustion engines to be automatically turned off by a control unit when the vehicle is at a standstill. This may help save fuel in a range from approximately 3% to approximately 5%.
Automatic transmissions, for example a stepped transmission, a dual-clutch transmission or continuously variable transmissions, are generally activated hydraulically and require a hydraulic pressure and a hydraulic volumetric flow for operation. The latter is provided by a mechanical pump (i.e., one driven by the internal combustion engine), the pump generally having an overdimensioned design due to the linear dependency of the volumetric flow on the rotational speed and due to reserves provided for taking into account the idling speed of the internal combustion engine and a possibly high oil temperature.
A system pressure controller sets a constant hydraulic pressure in the automatic transmission, and an excess amount of fluid is fed back into a tank or accumulator. Conventionally, the pump has a mechanically variable design (e.g., with the aid of an adjustment of the eccentricity of a vane pump), which may result in fuel savings.
When the internal combustion machine is stopped during the stopping phase, and when the hydraulic pump is stopped, the transmission may no longer be supplied with sufficient pressure or sufficient volumetric flow. Since the hydraulic circuit has certain leaks, the clutches and brakes are placed in an unpressurized (i.e., generally opened) position with the aid of restoring springs.
When the internal combustion engine restarts, it takes a certain amount of time before the mechanical pump generates enough pressure again. This results in a corresponding time delay before a starting torque is transmittable via the clutches. In addition, undesirable torque jumps may result when the clutches engage uncontrolled or slip. Furthermore, these clutches are generally not designed for the loads which occur.
To correct this, an electrically activated, on-demand oil pump may be used, which appropriately supplements the oil or hydraulic fluid in the transmission either continuously or shortly before the internal combustion engine starts. An alternative approach is to use an accumulator component. This component has the function of supplying an absent quantity of oil to the transmission shortly before or during the startup of the internal combustion engine, for the purpose of filling the lines and the transmission or the clutches.
In a conventional approach, a spring piston accumulator—for example, one having a capacity of approximately 100 ml (milliliters)—is mechanically latched in the filled state during the stopping phase and is charged by the hydraulic pump during normal vehicle operation. The charging point in time may not be influenced, since fluid flows from the transmission hydraulic circuit to the accumulator via a filling throttle as a function of the pump pressure as early as shortly after the engine starts up (i.e., at low rotational speeds).
During the stopping phase of the internal combustion engine, a solenoid of a valve controlling the fluid exchange is energized. Before and during the restart of the internal combustion engine, when the rotational speed rises continuously, the detent is released by de-energizing the solenoid, whereby a hydraulic pressure and an adequate quantity of fluid are provided for the transmission. The emptied cavities of the hydraulic circuit are filled, so that the pressure buildup by the mechanical pump takes place rapidly, and the motor vehicle may start up without a noticeable delay.
One general approach is to combine, e.g., any type of hydraulic accumulator (for example, a gas piston accumulator, a spring piston accumulator, a gas diaphragm accumulator having a barrier layer) in connection with an electrohydraulic valve (such as a 2/2-way valve). The accumulator is charged with fluid by the transmission oil pump during normal vehicle operation. During the stopping phase, the accumulator continues to store the fluid and may discharge it to the transmission or the hydraulic system shortly before and/or during the starting phase.
The valve must meet strict requirements with regard to tightness, fluid (medium) contamination and the required flow rate. For example, it may be required to achieve a flow rate of 30 liters per minute for a period of 200 ms (milliseconds). When charging the accumulator, it must also be kept in mind that the volumetric flow is limited, for example to approximately 3 liters per minute, so that the pressure in the transmission system does not drop due to the “absent” volumetric flow, or the mechanical transmission oil pump would possibly have to be more generously dimensioned.
The following describe systems in this field: DE 10 2006 041 899 A1, DE 10 2006 014 756 A1, DE 10 2006 014 758 A1, JP 10250402 A, U.S. Pat. No. 5,293,789 A1, EP 1 265 009 B1, US 20,050,096,171 A1, EP 1 353 075 A2 and JP 2007138993 A.